Can't use AT&T's Wi-Fi calling with your iPhone? This could be why [telecom]

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By Chris Ziegler

For the launch of the iPhone 6S, I elected to buy the T-Mobile version from Apple instead of the AT&T one, because I was planning on paying full price for the phone and the T-Mobile one comes unlocked out of the box. (I don't plan on leaving AT&T, necessarily, but an unlocked phone is easier to use internationally and has a higher resale value.)

I suspect there are some folks out there who did the same. What you may not have known, though, is that using this version of the phone on AT&T sets off a weird chain of events that you'll want to be aware of.